1. Field of the Invention
The present invention relates to an arithmetic unit for calculating electric power or electric energy based on voltage and current of a measurement system.
2. Description of the Prior Art
As an arithmetic unit for calculating electric power or electric energy in the prior art, there has been such an arithmetic unit as shown in FIG. 1, for instance. In FIG. 1, references T1, T2 denote input terminals to which respective voltages V1, A1 which are in direct proportion to voltage and current of a measurement system are input; 201, 202, first and second A/D converters for converting the voltages V1, A1 into respective digital values; and 203, a CPU which multiplies and integrates the digital values output from the first and second A/D converters 201, 202 in terms of software at intervals of a predetermined time. This arithmetic unit can calculate electric power W=V1.multidot.A1cos .PSI.. Therefore, generally, the arithmetic unit can function as a wattmeter if it integrates the electric power over more than one period and then changes integrated electric power into electric power per unit time, and also function as a watthour meter if it calculates the electric power in terms of infinite integral with respect to time.
In contrast, if reactive power should be calculated by the arithmetic unit, phase shift of 90.degree. is required since Q=V1.multidot.A1sin .PSI. has to be calculated. In the prior art, the CPU 203 has handled shifting process of the voltage V1 to calculate reactive power because the voltage V1 can be phase-shifted more easily in practical use.
However, in the case that both electric power and reactive power have to be calculated by the arithmetic unit in the prior art, there have been various problems as follows.
(a) It takes a lot of time to process multiplication instruction since multiplication has been executed in terms of software incorporated into the CPU. PA1 (b) Software is busy in calculation since multiplication has been executed in terms of software incorporated into the CPU, so that it is hard for the CPU to execute other tasks. PA1 (c) It is difficult to increase the sampling frequency since A/D conversion takes a lot of time because the A/D converters are employed. Also, if the bit number required for the A/D converters must be increased to improve conversion precision, it takes a lot of time to convert and in addition the arithmetic unit becomes expensive.
Further, particularly, in the case that reactive power is calculated, since data of the voltage V1 is phase-shifted in terms of software incorporated into the CPU, all 16 bits must be phase-shifted if the outputs of the A/D converters are 16 bit data, for example. There has been another problem that complicated process and excessive memories and registers are needed.